First, start by cutting slits into the pork, and stuffing sliced granny Smith apples into those slits.. It would be best to do this on a cutting board, it would be very difficult to do this in the slow cooker.
Next, lay onion and more apple slices down in the slow cooker, add a few pats of butter. You can do this for a bit of cooking liquid for the pork, and to add more flavor.
After adding the pork to the slow cooker, season with salt, pepper and cinnamon. Then, drizzle over honey, and lay over thyme sprigs. Cook this honey apple pork loin for 6 hours on low.